Google said the move means it will have capacity for 10,000 employees in the UK.
It currently employs more than 6,400 staff across the country.
Our focus remains on creating flexible workspaces that foster innovation, creativity and inclusivity
The Central St Giles office will undergo a multi-million pound refurbishment, to include meeting rooms for hybrid working, flexible working spaces and outdoor covered areas for people to work in the fresh air.
Google said it expected around a fifth of its workforce to work remotely as a result of changes to the world of work following the pandemic.
Senior Vice President Ruth Porat said: “We have been privileged to operate in the UK for nearly 20 years, and our purchase of the Central St Giles development reflects our continued commitment to the country’s growth and success.
“Our focus remains on creating flexible workspaces that foster innovation, creativity and inclusivity.”
Chancellor Rishi Sunak said: “This investment in jobs from Google is a big vote of confidence in the UK as a world-leading tech hub.
“It is also proof that this country continues to be one of the most attractive places in the world for leading firms to grow their business.
“The tech sector will play a pivotal role in our future economy and help drive forward our recovery from the pandemic, and we are committed to creating the right environment to ensure it continues to thrive.”
London’s Mayor Sadiq Khan said: “Google’s latest investment in London is fantastic news sending a clear message that our capital is a global draw for investment, business and talent. This will help London maintain its position as a world leading technology hub.”